Hand-held trimmers for cutting grass, brush and weeds are well known. Most hand held trimmers include a type of portable motor, a trimmer head having a rotating cutting component, such as a nylon cord or metal and plastic blades and a long boom arm. The trimmer head is connected at an angle to the distal end of the boom arm. In operation, a user typically grasps the boom arm or a handle part mounted to the boom at approximately the user's waist height and directs the rotating cutting component of the trimmer head at the distal end of the boom arm over the terrain. The length and angular orientation of the boom arm allows the operator to work the trimmer head directly over the terrain surface in a natural upright stance and without bending over. The angular orientation of the trimmer head with respect to the boom arm also positions the cutting plane of the rotating cutting component to be parallel to level terrain.
Many trimmer designs have been developed. Some trimmer designs incorporate the drive motor directly into the trimmer head, however, trimmers of this design are more difficult to manipulate and fatiguing to the operator because most of the trimmer's weight is located at one end of the boom arm. Consequently, most conventional trimmer designs locate the drive motors at opposite ends of the boom arms to provide more balanced and ergonomic operation. Since the motor and trimmer head are located at opposite ends of the boom arm, a drive shaft is rotatably disposed in the boom arm to transfer the rotational motion from the motor to the cutting component in the trimmer head.
Heretofore, lawn and brush trimmers that use a drive motor and trimmer head located at opposite ends of a boom arm have cutting planes that are fixed at permanent angular orientation with respect to the longitudinal axis of the boom arm. The angular orientation between the cutting plane and the boom arm is fixed due to the mechanical constraints of the drive shaft, which extends through the boom arm and connects the motor to the cutting components. The fixed angular orientation between the cutting plane and the boom arm make trimming difficult for persons of other than about average height. In addition, the fixed angular relationship can also make the trimmer difficult to use on uneven and sloping terrain. Consequently, in operation over un-level sloped terrain, an operator must hold and manipulate the trimmer to alter the natural balanced orientation of the boom arm in order to achieve a cutting plane parallel to the un-level sloped terrain. On upward sloping terrain, an operator must hold and manipulate the trimmer so that the trimmer head is raised and the boom arm is more parallel. On a downward sloping terrain, the operator must manipulate the trimmer so that the trimmer head is closer to the operator's feet and the boom arm is lowered at a more acute angle and the trimmer head is closer to the operators feet. Manipulating the trimmer outside of its natural balanced operating orientation makes the operation of the trimmer more fatiguing to the operator and may lead to unsafe use of the trimmer. Some lawn and brush trimmers have been developed that have boom arms that are adjustably connected to the trimmer heads, but these trimmers all locate the drive motor and trimmer heads at the distal end of the boom arms, which again creates an unbalanced tool for operation. Consequently, a balanced lawn and brush trimmer including an adjustment, which allows the attitude and orientation of the cutting plane of the trimmer head without changing the angular orientation of the boom arm from its normal balanced position is desirable.